Translation

Translation is the last arrow of the central dogma: DNA → RNA → protein. Transcription has already copied a gene into mRNA; translation is where that mRNA is turned into a protein. The single most important thing to hold onto is what gets read: the ribosome reads the mRNA, not the DNA. DNA stays in the nucleus; the mRNA carries its message out to the ribosome, and the ribosome works entirely from that mRNA copy.

The ribosome does not read the mRNA one base at a time. It reads it three bases at a time, and each three-base group is a codon. It moves along the mRNA in one fixed direction, 5′→3′, codon by codon, and builds the protein from its N-terminus toward its C-terminus. Reading begins at the start codon AUG and continues until a stop codon signals the end.

Hold onto two contrasts and the rest of the topic follows: what the ribosome reads (mRNA, three bases at a time — not DNA directly) and how the message is decoded (each mRNA codon is matched by a complementary tRNA anticodon carrying an amino acid). If you can answer those two questions, you will not misplace the DNA or confuse a codon with its anticodon.

Translation is a repeating cycle: read a codon, bring in the matching tRNA, join its amino acid to the chain, move on. Walking through one round makes clear what the ribosome reads and how a codon is matched to its anticodon.

1. Start with the mRNA — not the DNA. The ribosome binds the mRNA, the transcript that carried the gene's message out of the nucleus. From here on, the ribosome reads only the mRNA. The DNA is never fed into the ribosome; it stays behind in the nucleus.
2. Find the start codon, AUG. The ribosome scans to the start codon, AUG, and begins there. AUG both starts translation and codes for the first amino acid (methionine). Everything downstream is read from this point.
3. Read one codon — three bases at a time. The ribosome reads the mRNA in groups of three bases. Each triplet is a codon, and it moves along the mRNA 5′→3′, one codon at a time. A single codon specifies one amino acid.
4. Match the codon to a tRNA anticodon. A tRNA carrying an amino acid pairs with the codon through its anticodon — three bases that are complementary to the codon, not identical to it. Codon AUG is read by a tRNA whose anticodon is UAC; codon GCA by anticodon CGU. The codon (mRNA) and anticodon (tRNA) are different, complementary sequences.
5. Form a peptide bond and move on. The amino acid the tRNA delivered is joined to the growing chain by a peptide bond, extending the polypeptide from N-terminus toward C-terminus. The ribosome then shifts to the next codon. This repeats until a stop codon is reached, which carries no amino acid and ends translation, releasing the finished protein.

Notice the through-line: the ribosome reads mRNA (never DNA) three bases at a time, and every codon is decoded by a complementary anticodon on a tRNA that delivers one amino acid. AUG starts it, a stop codon ends it, and peptide bonds build the chain in between.

Quick reference card. For each term, read what it is and how it fixes what the ribosome reads or how a codon pairs with an anticodon — those two ideas are the whole game.

It is tempting to picture translation as the ribosome pulling letters straight off the DNA, or to imagine a codon and its anticodon carrying the same three bases. But the defining features are what the ribosome actually reads and how the codon is decoded. Feeding DNA to the ribosome — or conflating the codon with the anticodon — is where most points are lost.

The ribosome reads mRNA, not DNA. Translation still sits inside the central dogma: DNA → RNA → protein. Transcription already made the mRNA copy; translation reads that copy. The ribosome never touches the DNA — the DNA stays in the nucleus, and the mRNA is the only thing threaded through the ribosome. Saying the ribosome “reads the gene” or “reads the DNA” skips the mRNA and breaks the central dogma's directionality.

The reading is triplet and directional. The ribosome reads the mRNA three bases at a time, one codon per amino acid, moving 5′→3′ along the message. It starts at AUG and stops at a stop codon. The protein grows from its N-terminus toward its C-terminus, so both the mRNA reading and the protein building have a fixed direction.

Codon and anticodon are complementary, not identical. Each mRNA codon is matched by a tRNA anticodon — and the anticodon is the complement of the codon, not a copy of it. Codon 5′-AUG-3′ pairs with anticodon 3′-UAC-5′; codon GCA pairs with anticodon CGU. The tRNA that carries the anticodon also carries the amino acid, so complementary pairing is exactly how the right amino acid gets selected. Reading the anticodon as the same letters as the codon is the classic conflation error.

Keep the two questions straight. What does the ribosome read? (mRNA, three bases at a time, 5′→3′ — never DNA directly.) How is each codon decoded? (By a complementary tRNA anticodon carrying one amino acid.) Answer those and you will not put DNA in the ribosome nor make a codon and anticodon the same sequence.